A Chequer-Board of Nights and Days

Peter Watson says yes. Jane Galt convincingly begs to differ:

Monotheism: responsible for most of the wars in history. I was unaware that the early Romans, Greeks, Assyrians, Babylonians, Chinese, Japanese, Vikings, Mongols, Huns, Persians, Medes, Visigoths, and Zulu had all been monotheists. I was under the impression that they didn't really give a hoot what the people they conquered believed; they just wanted their stuff.

Perhaps he doesn't mean the majority of wars, but only the majority of deaths. This would tend to bias the results towards monotheism, simply because monotheism pushed into the far corners of the earth at the same time as the population explosion that followed the Industrial Revolution. But his definition doesn't really fit the Nazis, either, who could have cared less what the Slavs believed; all they cared about was bloodlines. Nor the parties in World War I, who didn't line up by any sort of recognizeable religious or cultural practice. And while I suppose one could categorize communist regimes, who are the other major source of body counts in the twentieth century, as believing in "one true God", none of them were notable for their emphasis the afterlife, unless you count badly executed bronze statues as "life after death".